MCP addConnection: fix lookup failure for top-level connectors (#15403)
When a connector name had no dot (e.g. "p3"), takeLast() emptied the parts list and getElementObject("") returned nullptr. Now single-part names are looked up directly in the diagram view instead of going through getConnectorElement.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
